1. Field of Invention
The invention relates generally to catalysts and methods of making catalysts and, more particularly, but not by way of limitation, to catalysts and methods of making catalysts that are useful for purifying exhaust gases and waste gases from combustion processes.
2. Background of the Invention
The high temperature combustion of fossil fuels or coal in the presence of oxygen leads to the production of unwanted nitrogen oxides (NOx). Significant research and commercial efforts have sought to prevent the production of these well-known pollutants, or to remove these materials prior to their release into the air. Additionally, federal legislation has imposed increasingly more stringent requirements to reduce the amount of nitrogen oxides released to the atmosphere.
Processes for the removal of NOx formed in combustion exit gases are well known in the art. The selective catalytic reduction (SCR) process is particularly effective. In this process, nitrogen oxides are reduced by ammonia (or another reducing agent such as unburned hydrocarbons present in the waste gas effluent) in the presence of a catalyst with the formation of nitrogen. Effective SCR DeNOx catalysts include a variety of mixed metal oxide catalysts, including vanadium oxide supported on an anatase form of titanium dioxide (see, for example, U.S. Pat. No. 4,048,112) and titania and at least the oxide of molybdenum, tungsten, iron, vanadium, nickel, cobalt, copper, chromium or uranium (see, for example, U.S. Pat. No. 4,085,193).
A particularly effective catalyst for the selective catalytic reduction of NOx is a metal oxide catalyst comprising titanium dioxide, divanadium pentoxide, and tungsten trioxide and/or molybdenum trioxide (U.S. Pat. No. 3,279,884). Also, U.S. Pat. Appl. Pub. No. 2006/0084569 (projected U.S. Pat. No. 7,491,676) teaches a method of producing an improved catalyst made of titanium dioxide, vanadium oxide and a supported metal oxide, wherein the titania supported metal oxide has an isoelectric point of less than or equal to a pH of 3.75 prior to depositing the vanadium oxide.
Vanadium and tungsten oxides supported on titania have been standard catalyst compositions for NOx reduction since its discovery in the 1970's. In fact, very few alternatives rival the catalytic performance of vanadium and tungsten oxides supported on titania. Despite the performance advantages of vanadium and tungsten oxides supported on titania, it would be advantageous to replace tungsten and/or vanadium with alternative metal components due to the significant drawbacks with using both tungsten and vanadium in SCR catalysts. First, tungsten shortages have led to increased costs associated with its use. Second, the potential toxicity of vanadium oxide has led to health concerns regarding its use in selective catalytic reduction DeNOx catalysts for mobile applications, as well as significant costs associated with disposal of spent catalysts.
It is known in the art that iron-supported titanium dioxide is an effective selective catalytic reduction DeNOx catalyst (see, for example, U.S. Pat. No. 4,085,193). However, the limitations to using iron as an alternative are its lower relative activity and, by comparison, a high rate of oxidation of sulfur dioxide to sulfur trioxide (see, for example, Canadian Pat. No. 2,496,861). Another alternative being proposed is transition metals supported on beta zeolites (see for example, U.S. Pat. Appl. Pub. No. 2006/0029535). The limitation of this technology is the high cost of zeolite catalysts, which can be a factor of 10 greater than comparable titania supported catalysts.
For implementation of lean burn engine technologies, the SCR DeNOx catalyst used must have the capability of achieving very high reduction of NOx over a broad range of temperatures, for example at least the range of 250° C. to 450° C. Most catalysts for lean burn applications exhibit satisfactory performance over only a fairly narrow temperature range; therefore, suitable catalysts are the focus of considerable research. Manganese oxide-based catalysts have been suggested for use as low temperature SCR DeNOx catalysts, as have similar iron, cerium, copper, tin oxide-base catalysts. However, the manganese oxide-based catalysts are limited at higher temperatures due to low ammonia selectivity. Another disadvantage when using manganese is its high selectivity for N2O formation, which contributes to ozone formation and acts as a greenhouse gas.
There remains a need for catalysts that exhibit improved performance for selective catalytic reduction of NOx in the presence of ammonia over at least a temperature range of 250° C. to 450° C. To this end, it is desirable to improve the ammonia selectivity of manganese, iron, cerium and/or tin containing SCR DeNOx catalysts at temperatures of 450° C. and above, while providing improved conversion activity at temperatures of 250° C. and below, as well.